Vehicles having positive power flow from the engine to the opposite drive units of a vehicle during steering without having to have duplicate pumps and motors on each side of the vehicle has gained in popularity. These vehicles have been generally called differential steer vehicles. A better understanding of the operation of vehicles having differential steer capabilities can be had by referring to U.S. Pat. No. 4,434,680 issued Mar. 6, 1984 to C. W. Riediger et al and assigned to Caterpillar Inc.
Since in differential steer vehicles steering is obtained by changing the relative speed between the opposite drive units, there is no way to ensure that one drive unit can be totally stopped in order to achieve a pivot turn. Under certain conditions, the operator may be able to achieve a pivot turn by making a steer input which provides a differential speed between the opposite drive units that matches the maximum predetermined differential speed of the differential mechanism. Under these conditions, one drive unit would be stationary and the other drive unit would be moving at the maximum differential speed. However, being able to select this exact position is quite difficult.
Furthermore, it is desirable to be able to selectively control the stopping of one of the drive units in order to achieve better control of the vehicle when maneuvering in tight areas or on slopes while still maintaining the advantage of power being continually available to each drive unit. Additionally, it is desirable in many applications to have the vehicle rotate or steer in the same direction regardless of vehicle travel direction. This means that upon the operator making a steer input to steer the vehicle counterclockwise, the vehicle will turn counterclockwise regardless of it being operated in the forward or reverse direction.
U.S. Pat. No. 4,307,796 which issued on Dec. 29, 1981 to Gary A. Hakes et al teaches a vehicle clutch and brake steering control system. In this arrangement, the vehicle is steered by interrupting power to one of the drive units by releasing a clutch and stopping the released drive unit by subsequently applying a brake. This allows independent braking of the opposite drive units of the vehicle but has the disadvantage of not having continual power available to both the right and left drive units during steering of the vehicle.
U.S. Pat. No. 4,541,497 which issued on Sept. 17, 1985 to Craig W. Riediger et al teaches a control mechanism for operating a tractor having differential steer. In this arrangement, a tiller bar is used to make a steer input by rotating the tiller bar about its vertical axis. Movement of the tiller bar in one direction initiates a vehicle steer which results in the vehicle turning clockwise while movement of the tiller bar in the opposite direction initiates a steer in which the vehicle is turning in a counterclockwise direction. Regardless of the vehicle traveling in a forward direction or in a reverse direction, movement of the tiller bar in the clockwise direction results in the vehicle turning in a clockwise direction. Likewise, movement of the tiller bar in a counterclockwise direction results in the vehicle turning counterclockwise regardless of whether the vehicle is traveling in the forward direction or the reverse direction. Even though this arrangement has a differential steer mechanism, there is no ability to selectively, independently apply the brakes on one of the drive units without applying the brake on the other drive unit.
U.S. Pat. No. 4,700,794 which issued Oct. 20, 1987 to James R. Bernhagen et al teaches a vehicle steering control apparatus for use on a vehicle having differential steer capability. In this arrangement, a steering wheel is used to make vehicle steer input and it was desirable to have the vehicle steer in the manner quite similar to the "C" type steering of an automotive type vehicle. Consequently, in order to provide a "C" type steer when going from forward to reverse direction, it was necessary to provide a switching valve to change the rotation of the steer motor which provides the differential control to the differential steer mechanism. In this arrangement, there is no selective, independent control of the brake on one drive unit independent of the brake on the other drive unit.
The present invention is directed to overcoming one or more of the problems as set forth above.